WASHINGTON—The most recent background check of former National Security Agency contractor Edward Snowden was so inadequate that too few people were interviewed and potential concerns weren't pursued, according to a federal review following his leak of some of the nation's most closely guarded secrets.

The background checkers failed to verify Mr. Snowden's account of a past security violation and his work for the Central Intelligence Agency, they didn't thoroughly probe an apparent trip to India that he had failed to...